Currently, I am developing a Flutter mobile app and implementing firebase push notifications in the app. Then I was wondering whether these notifications are charged in the case they fail to be delivered. For example, in the case the user deleted the app then his Firebase token is no longer valid, and therefore sending push notifications to that invalid token will fail. In this case, will I be charged for this failed notification? Thanks in advance.
I did some research about this but could not get a clear answer.
Then I was wondering whether these notifications are charged in case they fail to be delivered.
You won't be charged with anything, no matter if a notification arrives to the user or fails for any reason.
For example, in the case the user deleted the app then his Firebase token is no longer valid, and therefore sending push notifications to that invalid token will fail. In this case, will I be charged for this failed notification?
No, you won't. As @FrankvanPuffelen mentioned in his comment, the Firebase Cloud Messaging service is free of charge. There is nothing that you should pay.